Satellite communications (satcom) systems operating at microwave carrier frequencies (between 3 GHz and 300 GHz) typically employ parabolic reflector antennas through which signals from the satellite (downlink signals) are received while signals to the satellite (uplink signals) are simultaneously transmitted. The parabolic reflector is typically illuminated for both the uplink and the downlink by a single feed horn that supports all signal polarizations. In most satcom systems, the uplink signal frequency is somewhat higher than the downlink signal frequency. In typical Ku-band systems, for example, the uplink frequency is commonly in the range 14.0 GHz-14.5 GHz while the downlink frequency is commonly in the range 10.95 GHz-12.75 GHz.
In most satcom systems, the uplink signal polarization is specified by the communications system to be orthogonal to the polarization of the downlink signal in order to minimize cross-talk between the uplink and downlink signals. In these cross-polarized systems, the circular waveguide feed horn is connected to an orthomode transducer (OMT) to separate the orthogonally polarized uplink and downlink signals. The OMT may also include a transmit frequency band reject filter in the receive arm of the OMT. Each of the orthogonal arms of the OMT is in turn connected to an associated filter and to separate receiver and transmitter hardware in rectangular waveguide.
Some satcom systems are designed so that the uplink and downlink signals have the same polarization (i.e. they are co-polarized), even though associated signal cross-talk problems may thus be encountered. In one approach to antenna hardware for such a co-polarized system, the set of feed horn, OMT and filter hardware used for the more common cross-polarized systems may be easily adapted by inserting a co-polarization (co-pol) adapter (such as Prodelin TD0362 Co-Pol Adapter) between the feed horn and the OMT. The co-polarization adapter comprises a 90 degree waveguide twist and orthogonal waveguide filter arms that force the uplink and downlink signals to appear at the OMT to be orthogonally polarized signals. While this approach takes advantage of common waveguide components from the more-common cross-polarized system, the co-pol adapter adds considerable cost, weight, insertion loss and size to the feed assembly.
In a simpler alternative approach, a single-piece co-polarized diplexer (such as Prodelin TD0361 Co-Pol Diplexer) connected between the feed horn and the transmitter and receiver hardware is used to frequency-select the uplink and downlink signals and to connect to separate transmitter and receiver hardware. A co-pol diplexer uses waveguide filters and a waveguide junction to separate the (co-polarized) uplink and downlink signals presented to the co-pol diplexer in square waveguide and to feed separate transmitter and receiver hardware in rectangular waveguide. While the co-pol diplexer provides a fairly simple solution to the problem of uplink and downlink separation in a co-polarized satcom system, electrical performance requirements have dictated that known co-pol diplexer designs be manufactured using expensive electroforming or machining techniques because of the complex geometry required.
While known co-pol diplexers offer advantages relative to other techniques used for uplink and downlink signal separation in a co-polarized satcom system, significant cost is incurred to electroform or machine the co-pol diplexers of known designs. In many very small aperture terminal (VSAT) and other satcom terminal applications, the cost of a complex electroformed or machined part may be a significant proportion of overall satcom terminal cost. Therefore, while a single-piece co-pol diplexer has been developed, it is still desirable to develop an improved co-pol diplexer that does not suffer from the inherent limitations imposed by prior co-pol diplexer designs, such as disproportionate cost. Because large numbers of co-pol diplexers are needed in order to meet the needs of the global satcom market, there is a strong need for a co-pol diplexer that can be manufactured at a reduced cost.